The spread of cancer cells from a primary tumor to a site of metastasis formation involves multiple interactions such as invasion of extracellular matrix, neovascularization, invasion of the blood vessel wall (intravasation), exit from the circulation (extravasation) and establishment of secondary growth. Because cancer cells reach distant sites by disseminating through blood or lymphatic circulation, breaching of the vascular wall is a crucial event in metastasis formation. It is not known how early in disease progression cancer cells acquire the ability to intravasate. However, once established, this pathway appears to remain active as cancer cells can be detected in repeated blood samples of cancer patients.
Escape of cancer cells from the circulation, referred to as extravasation, is thought to be a major rate-limiting step in metastasis, with few cells being able to extravasate. However, highly metastatic cells are believed to extravasate more readily than poorly metastatic cells. A number of studies designed to assess metastasis by measuring extravasation have been performed using a chick embryo choriallantoic membrane (CAM) model system in which a large number of cancer cells are injected directly into the chorioallantoic membrane vein (Koop et al., 1994, Cancer Research 54:4791-4797; Chambers et al., 1997, J. Nat'l Cancer Inst. 84:797-803; Tsuchiya et al., 1993, Cancer Research 53:1397-1402; Shioda et al., 1997, AJP 150:2099-2112; Endo et al., 1990, J. Cancer Res. 81:723-6; Yamamoto et al., 1996, Anticancer Res. 16:413-7; Tsuchiya et al., Int. J. Cancer 56:46-51). However, since cancer cells are injected directly into the blood vessel, such studies fail to assay a tumor cell's ability to invade blood vessel walls (intravasation), a defect that would severely diminish the number of cancer cells delivered into the circulation and reduce the chances of metastatic growth.
While many aspects of cancer dissemination have been extensively studied, very little biochemical information related to the process of intravasation, i.e., the invasion of the blood vessel wall, is available. This may be in part due to the paucity of experimental models capable of mimicking the cellular and molecular interactions required for successful completion of intravasation. Existing models, such as mouse bladder (Poste et al., 1980 Cancer Res. 40:1636-1644) or human amnion denuded of epithelium and re-seeded with endothelial cells (Foltz et al., 1982, In "Interaction of Platelets and Tumor Cells", G. A. Jamieson, (ed.), New York: Alan R. Liss, Inc. pp. 353-371), are used infrequently because they recapitulate poorly the structure of blood vessels and in particular, small vessels such as capillaries and post-capillary venules, where most of the cancer cell invasion is believed to take place. The scarcity of "spontaneously" metastasizing human tumors in experimental animals is another obstacle to the study of intravasation. The reason for the inefficient "spontaneous" metastasis is not understood.
To identify properties that define cells capable of successful intravasation, and to screen for compounds capable of inhibiting metastic growth, it is imperative that quantitative in vivo assays are developed in which intravasation can be measured.